High seroprevalence epidemics among IDUs pose two critical inter-related questions for public health: how to halt ongoing HIV transmission among IDUs and how to halt the transition from a concentrated IDU epidemic to a generalized heterosexual epidemic. While there are several interventions that have been shown effective in reducing risk behavior among IDUs, no single intervention is effective for all IDUs. Thus, these inter-related problems provide an ideal context for studying combinations of prevention programs. There are five Specific Aims in the proposed research: 1.Conduct systematic reviews of social structural and behavioral interventions to reduce injecting and sexual risk behavior, and biomedical interventions (drug abuse treatment, STI treatment) to reduce risk behavior among injecting drug users, and interventions to build community and policy support for HIV prevention among IDUs. For interventions with previous systeml interventions to reduce injecting and sexual risk behaviors among IDUs and of combinations of interventions at different levels of coverage, including optimization strategies for reducing HIV transmission among IDUs and from IDUs to non-drug using sexual partners 3. Monitor the ongoing high seroprevalence HIV epidemics among IDUs in Tallinn, Estonia to assess current interventions and model an optimal set of interventions for that epidemic. HIV prevalence is currently over 50% among IDUs in Tallinn, estimated incidence is over 20/100 person-years at risk and IDUs report high rates of multiple sexual partnerships with inconsistent condom use. 4. Conduct a pilot study of screening and treatment for sexually transmitted infections (STIs) among IDUs in Tallinn to assess feasibility and effectiveness in reducing sexual transmission among IDUs and from IDUs to non-drug injecting sexual partners. The pilot study will include biomedical intervention (screening and treatment) with a state-o transmission from IDUs to non-drug injecting sexual partners are two critical public health problems in many different countries. These inter-related problems are an ideal context for studying the effects of combined HIV prevention programs. Public health significance: Reducing HIV transmission among IDUs and preventing transmission from IDUs to non-drug injecting sexual partners are two critical public health problems in many different countries. These inter-related problems are an ideal context for studying the effects of combined HIV prevention programs.